When broken down to the basics, a modern refinery has the overall function of taking a naturally-occurring material--crude oil--and creating a multitude of other more directly useful products such as gasoline, kerosene, lubricating oils and the like. The refinery accomplishes its purpose by two major operations: separation of hydrocarbons and by conversion of hydrocarbons.
The separations may merely remove one particular kind of hydrocarbon from a mixture; for instance, kerosene is simply distilled from crude oil. Or the separation may remove an undesirable material from other useful materials; for instance (as in the current invention), a metal-containing compound may be separated from other oils.
The other generic function of the refinery is the conversion of hydrocarbons. This may entail changing the type of or the molecular weight of the oil. For instance, the catalytic reforming process begins with a hydrocarbon feedstock which is largely made up of straight-chain hydrocarbon molecules (e.g., alkanes such as hexane) and converts them into materials which are closed chains (e.g., aromatics such as benzene or xylene). Aromatics tend to make higher-octane gasolines than the reformer feedstocks. Alternatively, the refinery process may change the size of the hydrocarbon molecule. Catalytic cracking involves the use of a catalyst to break or "crack" larger oil molecules into smaller molecules suitable for use in final products such as gasoline. Cracked materials may also be used as feedstocks to other refinery processes.
However, the overall refining process is not a static one. Market forces make demands on the products required of the refinery and also define the types of crude oils available as feedstocks to the refinery. Over the past dozen years, various conservation measures have changed the amount of gasoline needed in the marketplace. The requirement in the U.S. that lead compounds be substantially removed from gasoline sold for use in automobiles has caused similar massive changes in the way gasoline is produced. The decreasing availability of so-called "light" crude oils, those which typically contain less tar-like materials and sulfur-bearing compounds, has led to the major redesign of many refineries in the U.S. to allow use of the more-available heavier oils.
The use of heavier oils has had several immediate and clear consequences in the overall refinery process. First of all, the heavier oils contain certain contaminants not as prevalent in the lighter oils. These contaminants include sulfur and nitrogen compounds as well as metal-bearing compounds such as the nickel and vanadium contaminants removed by the present invention. As an additional result of the heavy crude use is the increased emphasis on refinery processes which convert heavier hydrocarbons to smaller and more useful ones. A complicating factor is that a substantial number of the additional contaminants are harmful to the catalysts used in the now more important conversion processes.
The metal vanadium, in particular, tends to deposit on the catalytic cracking catalyst and tends to increase both the production of coke (which coats portions of the catalyst rendering it less useful) and the production of hydrogen. Hydrogen is not generally seen as a useful end product. Vanadium also attacks the cracking catalyst itself, in particular the active zeolite portion of the catalyst, apparently as a migrating vanadium oxide material.
Both nickel and vanadium-bearing compounds are additionally undesirable if left in the refinery products which are separated, or distilled, from crude feedstocks. Fuel oils containing these metals may be harmful to fired boilers. The vanadium and nickel compounds create corrosive and persistent deposits in the cool ends of the boilers. The deposits often must be removed by hand.
In any event, processes which remove the nickel and vanadium compounds present in crude oils (for its various fractions) are clearly becoming more desirable.
The methods used in the past included one in which the oil was treated with a non-oxidizing acid such as hydrogen chloride. The metals in the oil precipitated in the form of an "acid sludge". The treated oil was then separated from the sludge, neutralized and fractionated to remove lower boiling constituents. The remaining fraction containing quantities of nickel and vanadium was then catalytically cracked and the spent catalyst (containing large concentrations of nickel and vanadium) was demetallized by extraction before being recycled to the cracking stage.
The metals in many of the heavier oil fractions have been found to be present in the form of low molecular weight chelates, particularly as metalloporphyrins. It is said that these metals may be removed by extraction with physical solvents such as gammabutyrolactone, acetonitrile, phenol, furfural, 2-pyrrolidinones, dimethyl sulfoxide, dimethylformamide, and pyridine-water mixtures. Gammabutyrolactone is said to be the best among these prior art solvents in respect to selectivity for vanadium and nickel. See, U.S. Pat. No. 2,913,394 by C. N. Kimberlin and W. J. Mattox.
The present invention may utilize the prior art solvents noted above in the metal extraction step or, more preferably, certain polar solvents which are superior to gammabutyrolactone when used as solvents for extraction of vanadium and nickel metalloporphyrinic compounds from oils. Among the solvents which are among those preferred are ethylene carbonate, propylene carbonate, ethylene thiocarbonate and dimethyl sulfone. The solvents are regenerated either by the step of extracting the metal-containing compounds with a highly aromatic oil or by extracting the metal-containing compounds with a highly aromatic oil while cooling the back-extraction step.
Certain of these solvents, of course, have other uses. As is shown in U.S. Pat. No. 4,348,314, ethylene carbonate is used as a specialty solvent for polymers. As is further shown in U.S. Pat. No. 3,018,228, it has also been used for aromatics extraction and the synthesis of pharmaceuticals. U.S. Pat. No. 3,912,801 discloses the extraction of a number of metal-bearing compounds from acid aqueous media using an alkylene carbonate.
None of the known prior art shows an integrated process for the separation of nickel and/or vanadium bearing metalloporphyrinic compounds from oils by using the specified solvents and regenerating those solvents for recycle to the metal extraction step.